Synchronous detector with pulse repetition frequency modulation

ABSTRACT

A moving target indicating radar that includes a coherent on receive system and a clutter referenced velocity compensator operating in association with a linear predictor to produce coherent IF (intermediate frequency) receive signals shifted in frequency to compensate for the radar platform&#39;s motion. A synchronous detector, with pulse repetition frequency modulation, translates the coherent IF received signals to video signals at a desired frequency offset. A storage processor processes and range gates the video signals prior to doppler filtering and displaying the signals as a representation of moving targets.

This invention relates to radar systems and particularly to a pulse repetition frequency modulator operable in a radar system for providing improved and reliable signal processing of the radar signals.

For detection of moving targets such as from an aircraft moving over terrain of varying configurations, delay line techniques have been conventionally utilized in either coherent or noncoherent type radar systems. As these moving target indicators operate with sweep-to-sweep radar signal comparisons, extremely close control of the radar pulse repetition rate is required. Also, these systems are limited in their performance at low pulse repetition rates and high platform speeds because of the selectivity characteristics of the comb filters which discriminates against clutter signals. Noncoherent systems utilizing delay line techniques operating with sweep-to-sweep signal comparisons also require clutter to be at the same range and relative bearing as the target for satisfactory operation. Noncoherent moving target techniques are utilized because of their relative freedom from platform motion effects but their dependence upon target-to-clutter ratios restricts their performance. Conventional clutter referenced coherent systems have been found to be undesirable because of momentary discontinuities of clutter energy and a tendency to unlock on moving targets, especially when operating over rough terrain.

Radar systems which operate at a high pulse repetition rate are ambiguous in range determination and are highly sensitive to side-lobe clutter signals when airborne at intermediate and low altitudes. Thus, high pulse repetition radar systems are limited in their performance against targets with beam to tail aspects with respect to radar line-of-sight and against targets moving at low velocities over terrain which provides strong main lobe clutter signals. Most pulse doppler radars operate at a high pulse repetition frequency to avoid the doppler blind speeds which occur when the separation of the repeating spectral lines are less than the relative doppler shift between the clutter and the target.

A pulse doppler radar using a low pulse repetition frequency (as with conventional noncoherent radar systems) provides unambiguous range information but the velocity information is ambiguous and part of the spectral region is obscured as well. This type of system may utilize range gating to provide a desirable signal-to-clutter ratio and to prevent clutter signals from all ranges from collapsing into the filters. Blind periods, during which the target passes into the clutter spectral region, can be eliminated by sensing the occurrence of velocity eclipsing and varying the pulse repetition frequency or by suitable programming. A system that is coherent, operates at a low pulse repetition frequency to minimize side lobe and main lobe clutter effects and to maximize performance with respect to target aspect and speed, utilizes velocity compensation to minimize the sensitivity to clutter signal frequency fluctuations and amplitude fluctuations and processes signals matched in frequency characteristics to the transmitted signal, would be highly desirable in the radar field. A system that utilizes storage or scan conversion devices for processing signals would be highly desirable for improved doppler filtering and target detection.

In a radar system in which the video data is stored in the scan conversion devices such as storage tubes, it is desirable to position certain elements of the spectrum at a desired frequency rather than at DC as in conventional synchronous detectors.

When a radar system has a variable PRF and a signal prior to synchronous detecting to a video signal, that varies in frequency, a mixer responsive to the input signal and an offset reference frequency is undesirable because of the required circuit complexity. In such an arrangement an oscillator and single sideband modulator must shift the reference frequency by exactly 1/2 PRF, for example, and control during varying input conditions and changing PRF requires relatively complex circuits.

It is therefore an object of this invention to provide an improved and simplified synchronous detector for providing a desired frequency offset.

It is a further object of this invention to provide an improved synchronous detector circuit in which a spectral signal being a function of the pulse repetition frequency is provided with a reliable frequency offset as a function of the pulse repetition frequency.

It is a still further object of this invention to provide an improved video detector that reliably positions a signal being a function of the pulse repetition frequency, to a predetermined frequency.

It is another object of this invention to provide a system for modulating target signals to a relatively low frequency spectral position.

Briefly the synchronous detector system in accordance with the principles of the invention includes a mixer or cosine φ phase detector responsive to an input signal having spectral signals such as clutter signals repetitive at pulse repetition frequency intervals. A reference oscillator is maintained at substantially the same frequency as a selected clutter signal. To provide a desired frequency offset to the reference frequency, a balanced modulator arrangement is coupled between the reference oscillator and the mixer. During each pulse repetition frequency interval, the phase is changed in the balanced modulator arrangement a selected amount to provide a desired frequency offset. The input signal spectrum is folded at a pulse repetition frequency point as a function of the frequency offset of the reference signal to develop a video signal at DC with the clutter signals at frequencies determined by the folding point. Thus, for example, when the reference signal is shifted in phase 180° each pulse repetition frequency interval, the spectrum of the input signal folds at 1/2 PRF and the selected clutter signal is at 1/2 PRF regardless of PRF changes because the PRF offset is developed in response to the synchronous PRF intervals.

The novel features of this invention, as well as the invention itself, both as to its organization and method of operation, will best be understood from the accompanying description, taken in connection with the accompanying drawings, in which like reference characters refer to like parts, and in which:

FIG. 1 is a drawing showing the operation of the illustrated doppler system to detect a moving target flying over terrain of varying reflection characteristics;

FIG. 2 is a schematic block diagram of the illustrated doppler moving target detection system;

FIG. 3 is a schematic block diagram showing a first portion of the radar system of FIG. 2;

FIG. 4 is a schematic diagram of one of the storage tubes that may be utilized in the storage processor of FIG. 2;

FIG. 5 is a schematic diagram of the other storage tube that may be utilized in the storage processor of FIG. 2;

FIG. 6 is a schematic block diagram of another portion of the doppler processing system of FIG. 2;

FIG. 7 is a schematic block diagram of the processor timing control system that may be utilized in the system of FIG. 2;

FIG. 8 is a schematic diagram further illustrating the timing control system of FIG. 7;

FIG. 9 is a schematic diagram of a portion of the timing control system of FIG. 7;

FIG. 10 is a schematic circuit diagram of a phase rotator that may be utilized in the system of FIG. 3 to convert noncoherent received signals to coherent signals;

FIG. 11 is a schematic circuit diagram of the space to aircraft coordinate resolvers that may be utilized in the system of FIG. 2 for developing the aircraft velocity function along the antenna line-of-sight;

FIG. 12 is a schematic circuit diagram of the aircraft coordinate resolvers that may be utilized in the system of FIG. 2 for developing the aircraft velocity rate signals along the antenna line-of-sight;

FIG. 13 is a schematic circuit diagram of the resolvers responsive to the aircraft velocity vectors to develop signals representative of the velocity along the line-of-sight of the antenna;

FIG. 14 is a schematic circuit diagram of the resolvers responsive to the aircraft velocity rate vectors to develop signals representative of the rate of change of line-of-sight velocity with antenna angle in a horizontal plane;

FIG. 15 shows schematic vector diagrams looking from the side and from the tail of an aircraft to show the space stabilized antenna angles;

FIG. 16 shows schematic diagrams of the aircraft references axes looking from the top and from the side of the aircraft;

FIG. 17 is a schematic circuit diagram showing the phase selection circuit of FIG. 6 for correcting the target signals to have a narrow spectral width;

FIG. 18 is a schematic circuit diagram of the step attenuator that may be utilized in the system of FIG. 6 to correct the spectral width of the target signal;

FIG. 19 is a schematic block diagram of the correlation oscillator of FIG. 6;

FIG. 20 is a schematic circuit diagram of one example of a wide band mixer that may be utilized in the system of FIG. 6;

FIG. 21 is a schematic diagram showing the antenna lobes relative to a moving radar platform, a target and the reflectors adjacent or below the target for explaining the spectral width of the main lobe clutter signals;

FIG. 22 is a spectral diagram showing the folding of the radar signals around a one-half PRF (pulse repetition frequency) point and showing the frequency modulation of the clutter signals to a frequency equal to one-half PRF in accordance with the principles of the invention;

FIG. 23 is a schematic spectral diagram showing the operation of the clutter filter and discriminator in the system of FIG. 3;

FIG. 24 is a schematic diagram of percent response versus frequency of the storage tubes that may be utilized in the system of FIG. 2;

FIG. 25 is a schematic diagram showing the positioning of the doppler data on the screen of the storage tubes in the system of FIG. 2;

FIG. 26 is a schematic timing diagram showing wave forms of time versus voltage for the recording and reading operation of the storage tubes;

FIG. 27 is a schematic timing diagram including wave forms of voltage versus time for further showing the recording and reading operation on the storage tubes;

FIG. 28 is a schematic timing diagram of time versus voltage and time versus frequency for further explaining the operation of the system of FIG. 2; and

FIG. 29 is a schematic circuit and block diagram of another arrangement of the pulse repetition frequency modulator in accordance with the invention.

Referring first to FIG. 1, a radar system is illustrated in which the PRF (pulse repetition frequency) modulator of the invention may operate in an aircraft or platform 10 over relatively rough surfaces such as ground or water surfaces to detect low flying aircraft such as a craft 12 or to detect slowly moving ground targets. However, it is to be understood that the radar system is not limited to operation in any particular type of terrain or to operation from a moving platform or aircraft. The system may operate with a relatively low pulse repetition frequency to reduce the intensity of return clutter energy and to reduce the effect of side lobe clutter return signals. It is to be noted that the system may operate at low, medium and high PRF such as over a PRF range of 100 pulses per second to 50,000 pulses per second, for example. Transmitted pulses of energy of a waveform 14 may be derived from a magnetron having random starting phases. The reflected energy, which may include doppler shifted target echo signals of a waveform 18 and doppler shifted ground echo signals or main lobe clutter signals of a waveform 20, is then corrected in phase to provide coherent operation. In the low pulse repetition frequency system, range gating may be performed so that clutter energy is only passed into the system from a range interval including the target 12. Although the system of the invention is illustrated for tracking slow or fast moving targets, wither in the air or on the surface such as ground or water, and in the presence of varying clutter return energy, it is to be understood that the principles of the invention are equally applicable to radar type systems operating as ground based radar for detecting either slow or fast moving targets in space or over or on any type of surface.

Referring now to FIG. 2, the system of the invention may operate with a radar transmitter and receiver system 25 which may be either a coherent system or a noncoherent system with an arrangement for developing coherency upon signal reception. If a noncoherent radar system 24 is utilized which may include a magnetron or other random phase source of energy, a coherent or receive system 26 may be utilized to render the phase of the signals of sequential pulses coherent, that is, the signals of each sequential pulse have a known phase relation. The coherent IF (intermediate frequency) signals are applied from the coherent system 26 to velocity compensation circuits 28 which establishes target doppler sensitivity with respect to stationary clutter echoes in the vicinity of the target. The coherent IF signals are compared to a reference frequency of a variable frequency oscillator to develop corrections which are summed with predicted correction signals developed by a linear prediction circuit 30 so as to operate reliably during periods of a relatively small clutter return or the absence thereof. A navigator 32 applies aircraft velocity signals in space coordinates to the linear predictor circuit 30 and an antenna resolver circuit 34 applies antenna directional information in aircraft coordinates to the predictor circuit to predict the line-of-sight velocity V_(LOS). The navigator 32 may be either a stable platform and an air data computer, an inertial guidance system, a doppler navigation system, or any type of system for developing velocity vector signals in space coordinates. Also, the principles of the invention are applicable to a system in which inertial guidance systems and digital or analog computers develop signals representative of velocity and antenna vectors.

The coherent IF signal is then applied from the velocity compensation circuit 28 to a modulator 29 to form a clutter referenced video signal with the centroid of the clutter spectrally shifted to a desired one-half PRF frequency so that the target signal appears over a range of low frequencies. The video signal is then applied to a storage processor 36 which may include a scan conversion device such as an electrical storage tube, a dielectric belt, a magnetic tape or a digital processing device in which the data is recorded in a line-scan fashion forming a field of range elements in a first dimension and time in a second orthogonal dimension. The stored data is then read in the orthogonal dimension (horizontal in the illustrated system) so that a plurality of contiguous range elements are sequentially examined. The storage device selected may have frequency characteristics responsive to the spectral region of the target signal and relatively unresponsive at higher frequency bands, thus providing filtering. Range gating is performed in the storage processor 36 and a signal including clutter signals at known frequencies and target signals at doppler frequencies due to the motion of the target over stationary ground, is applied to a clutter filter 40. The passband of the filter 40 which is a variable bandwidth filter, is controlled to pass those frequencies not occupied by clutter signals in response to a process control circuit 42. The clutter spectrum is a function of the aircraft velocity and the antenna look angles such that a signal proportional to the rate of change of line-of-sight velocity with antenna angle in a horizontal plane (V_(LOS)) is utilized to control the clutter filter.

The target signal spectrum after removal of clutter signals is applied from the clutter filter 40 to a target spectral control circuit 44 where the signal is mixed with a swept oscillator to remove the predictable frequency modulation imparted to the target signals as a result of the velocity compensation process. The signal V_(LOS) controls this frequency correction to develop frequency modulations of first and second phases during two respective readings of the stored data, one correcting the frequency modulated target signal to its original spectral width. The corrected target signals are then applied to a bank of doppler filters and detectors of a circuit 48 and in turn to a utilization device such as an azimuth-range display circuit 50 or to other processing circuits such as a computer or correlator.

Referring now to FIG. 3, the illustrated coherent radar system may operate in conjunction with the noncoherent system 24 utilizing a source of signals of random phase such as a magnetron transmitter 54. The principles of the invention are also applicable to operation in conjunction with other types of transmitters and receivers such as phase locked coherent arrangements. The magnetron transmitter 54 may be maintained at a frequency f_(x) or within a fraction of an MC (megacycle) of that frequency by a feedback circuit including an i-f discriminator 56 that generates a signal amplitude proportional to frequency variations to control a tuning servo 58. The magnetron transmitter 54 responds to a signal developed by the tuning servo to correct the frequency thereof to essentially that of the i-f discriminator 56. The transmitted energy is applied from the transmitter 54 through a duplexer 60 to the feed of an antenna 62 to be transmitted into space at an angle relative to the aircraft velocity vector determined by a scan control mechanism 64. Suitable devices may be included in the mechanism 64 to provide information as to azimuth antenna pointing angle η and vertical antenna pointing angle ξ relative to the aircraft axes. The transmitted energy may be reflected from reflectors such as targets and surfaces such as terrain, ground structures and water and is intercepted by the antenna 62 with a frequency f_(x) + f_(ds) where f_(ds) is the doppler shift caused by target movement and by the motion of the platform or antenna 62 relative to the illuminated patch on the earth's surface. The magnetron transmitter 54, the discriminator 56, the tuning servo 58, the duplexer 60 and the antenna 62, which are devices well known in the art, may be included in a conventional low PRF noncoherent radar system 24, which may be utilized as a portion of the system in accordance with the invention. An advantage of a magnetron transmitter is the high power capability and the relatively simple control circuits.

Because the magnetron RF (radio frequency) signal is uncorrelated from pulse to pulse, a sample of the starting phase of each transmitted pulse is applied from the magnetron through a conductor 66 to a mixer 68 for phase comparison at RF frequencies with the signal developed at a stable reference frequency f_(x) + f_(o) by a local oscillator 70. It is to be noted that the sample signal may be derived from other system locations such as utilizing the leakage energy from the duplexer 60. A stable reference oscillator 72 which provides the reference phase at IF frequency f_(o), is applied to a phase detector 74 as well as through a 90° phase shifter 76 to a phase detector 78. The sampled signal at IF frequency is applied from the mixer 68 to the phase detectors 74 and 78 to develop error signals having DC amplitudes for each transmitted pulse corresponding to the instantaneous phase error between the reference oscillator 72 and the IF signal derived from the sampled magnetron RF signal. The error signals are stored in memory means such as box car circuits 80 and 82 for one interpulse period being gated by a PRF synchronizing pulse from radar control circuits 79 which may be included in the noncoherent radar system 24. The box car circuits 80 and 82 may be conventional diode ring types such as shown in an article by Paul E. Harris, "Radar Boxcar Circuits Using Nuvistors, Transistors and Zeners," in Electronics, Sept. 16, 1961, pages 66 to 67. Each box car circuit may include zener diodes to normally back bias semiconductor diodes and disconnect a capacitor from the source. A blocking oscillator pulse or radar synchronizing pulse from the transmitter provides a large circulating current through the diodes. The phase sensitive pulse from the phase detector diverts current to the capacitor to establish the stored error signal. A clamp signal may be utilized in response to a pulse (not shown) developed during the radar transmission time.

The signals stored in the box car circuits 80 and 82 are applied to a phase rotator 84 which may include a pair of suppressed-carrier, balanced modulators driven in phase quadrature to develop a single side-band modulated signal. A mixer or cosine φ phase detector 86 responds to the phase correction signal of the form cos (wt + φ) and to an IF signal f_(o) - f_(ds) developed by a mixer or phase detector 88 which in turn responds to the local oscillator signal at frequency f_(x) + f_(o) and the signal at f_(x) + f_(ds) received from the duplexer 60. The mixer 86 applies a coherent signal f₁ - ΔS to a lead 90, which signal is applied through a range gate circuit 92, a signal actuated gate 67 and a narrow band filter 94 having a frequency centered at f₁, to a lead 96. The term ΔS represents the frequency error that is operating in a closed loop 95 of the velocity compensation circuit 28. A discriminator circuit 98 centered at the frequency f₁ applies the coherent IF signal through a switch 100 and a function generator circuit 102 to a summing circuit 104. The circuit 102 may include an amplifier 101 and a feedback path including a capacitor 103 and a resistor 105 to develop a low pass filter characteristic. A voltage controlled oscillator 108 operates at a frequency f_(o) - f₁ - f_(ds) + ΔS in response to the summing circuit 104 to apply the reference signal to the phase rotator 84. The loop 95 is a velocity compensation loop which measures the error between the frequency f₁ and the corresponding frequency of the VCO 108, for removing the doppler shift of the clutter signal, that is, maintaining the selected clutter spectral signal at the frequency f₁ or at f₁ - ΔS, where ΔS is a small error. The switch 100 closes the loop when the prediction voltages have connected themselves or reached stable values such as during the start of a radar operation, and allows the loop 95 to lock into the central line of the clutter spectrum, that is, the clutter line in which the apparent doppler shift is insensitive to the pulse repetition frequency. The switch 100 may be operated either manually or electronically.

The range gate 92 which may be of a type well known in the art, may have a manual range control or may be controlled as a function of the expected terrain clearance and the antenna depression angle. It is to be noted that although the system may operate to pass the central line of the clutter at frequency f₁, the principles of the invention are applicable to passing other clutter lines at the frequency f₁.

The signal actuated gate 67 includes a transistor 71 having a transformer tuned as a collector load. Two diodes are provided which are forward biased during static conditions. The input signal is also passed to an amplifier and threshold detector 73 which senses the peak signal level so that when the signal reaches the threshold of the detector a video signal is applied to the base of transistor 69. As a result, the transistor 69 conducts to open the two diodes. The impedance at the collector of the transistor 71 is then raised to a relatively high value and the clutter spectral signal is passed to the narrow band filter 94. For signals having less amplitude than the threshold, the gain is very low and for high level signals the gain is raised. The threshold may be set to pass signals approximately 6 decibels, for example, above random receiver noise. The signal actuated gate 67 allows the open loop or the linear predictor 30 to operate in the absence of clutter signals without the closed loop tracking the noise signals.

The discriminator 98 has a voltage versus frequency characteristic (FIG. 23) to pass the central line of the spectrum and attenuate the remaining spectral lines, thus integrating all clutter signals passed through the range gate 92. The discriminator 98 provides an error signal as a second order correction and the linear predictor circuit 30 provides first order corrections in response to the antenna resolver circuit 34 and aircraft velocity vectors V_(I), V_(J) and V_(K) derived in space coordinates from the navigator 32 or other suitable source of velocity signals. For example, the navigator 32 may be an inertial type guidance system having a gyro and sufficient gimbals to develop velocity vectors or may be a doppler navigation system as is well known in the art. For process control in the system, such as controlling the passband of the processor and correcting the spectral width of the target signals, a process control circuit 42 responds to the velocity vector signals from the navigator 32 to develop a signal V_(LOS) representing rate of change of velocity along the antenna line-of-sight or δV_(LOS) /δη.

Doppler navigation systems are well known in the art as described on page 103 of "Introduction to Radar Systems" by Merril I. Skolnik, Mc-Graw-Hill Book Company, Inc., 1962 and as described in an article, "Navigating the Supersonic B-58, " by Capt. William L. Polhemus, USAF, in Journal of the Institute of Navigation, Summer-Autumn 1960, Vol. 7, Nos. 2 and 3. Inertial navigation systems are well known in the art as described in Chapter 3 of "Principles of Inertial Navigation," by C. F. Savant, Jr., R. C. Howard, C. B. Holloway and C. A. Savant, by Mc-Graw-Hill Book Company, Inc., 1961 and in an article, "Effect of Component or Orthogonality on Aircraft Inertial Navigation System Errors," by M. Kayton and R. C. Gilbert in the Proceedings of the 1st American Institute of Aeronautics Astronautics Annual Meeting, Washington, D. C., June 29 - July 2, 1964.

The signal on the lead 90 corrected for clutter motion by maintaining the central line of the clutter spectrum centered at f₁, is applied to a mixer or cosine φ phase detector 120 which applies a clutter referenced video signal to a lead 122. A VCO (voltage controlled oscillator) 124 centered at frequency f₁ applies a signal to a one-half PRF modulator 126 also responding to a flip flop 127 being triggered by a radar synchronizing or PRF pulse. A phase detector 130 responds to the signal on the lead 96 and to the signal developed by the VCO 124 to apply a correction signal through a gain control circuit 132 to the VCO 124, to maintain the VCO phase locked with the signal on the lead 90. The circuit 132 may include an amplifier and feedback path including a series coupled capacitor and resistor coupled in parallel with a capacitor. In one arrangement in accordance with the invention, the modulator 126 may include a transformer 131 having a first winding coupled to the VCO 124 and a second winding with a center tap coupled to an output terminal of the flip flop 127. Oppositely poled diodes 133 and 135 may be coupled to the opposite ends of a second winding of the transformer 131 and through an inductor and capacitor to ground as well as to the mixer 120. The modulator 126 may operate in response to a phase change at every interval of the pulse repetition frequency to provide a 180° PRF phase change and a 1/2 PRF frequency offset (increase or decrease) of the signal developed thereby so that the central line of the clutter spectrum is centered at a frequency of 1/2 PRF when folding in the mixer 120 to DC. Thus in accordance with the invention the target signal is established within the region of desirable frequency response of the storage device of the processor.

The clutter referenced video signal on the lead 122 is then applied to write amplifier and bias circuits 164 and 164a of FIGS. 4 and 5 for controlling the intensity grids of storage tubes 136 and 138. The storage tubes in the illustrated system may be any conventional recording storage tube such as Raytheon CK7571/QK685. It is to be noted that the principles of the invention are not to be limited to use of storage tubes or to any particular type of storage tubes but any suitable storage device may be utilized such as magnetic tapes or surfaces or digital type memory systems. The illustrated storage tube 136 of FIG. 4 has a collector 140, a storage surface 142, a decelerator grid 144, a cathode 146, an intensity grid structure 148 and other control grids, collimating lenses and anodes controlled from a high voltage supply circuit 139 as is well known in the art. The tube 136 is utilized with a vertical deflection dynamic focus (DF) coil 150, a horizontal deflection focus coil 153, a vertical deflection coil 152, a horizontal deflection coil 154 and a static focus coil 151, all of which may be conventional type deflection yokes. The storage tube 136 is utilized with four typical operations; prime, write, read and erase. Priming is accomplished by uniformly charging the storage surface with the screen voltage reduced below a critical potential while the raster is scanned. During writing, a suitable quiescent (mid-gray scale) bias and a signal of desired amplitude controls the intensity grid. During reading, the grid bias is set to provide a suitable unmodulated beam utilized to scan the raster and resulting in the stored signal forming a beam modulation which is detected at the collector. During erasing, a DC signal is written into the tube to normalize the storage surface at full modulation level. The tube 138 of FIG. 5 includes similar components and functions in a similar manner.

The write amplifier and bias circuit 164 responds to the clutter referenced video signal to apply a modulation to the intensity grid 148. A source of bias potential 166 controls the grid bias in response to timing signals W₁, R₁, E₁ and P₁, respectively representing write, read, erase and prime, which may be decoded in a coding circuit 168 for selecting the desired bias potential during each operation. An unblanking pulse is also applied to the coding circuit 168. A static focus supply 172 applies vertical deflection signals to the static focus coil 151. For providing focusing as a function of the vertical sweeps, a read vertical dynamic focus generator 189, and a write vertical dynamic focus generator 188 apply signals through an OR gate 187 to a vertical deflection amplifier 186 and in turn to the vertical focus coil 150. During writing, a horizontal dynamic focus generator 178 applies a signal through an OR gate 167 and a horizontal deflection amplifier 174 to the horizontal focus coil 153. During reading, a horizontal DF sweep generator applies a dynamic focus signal through the OR gate 167 to the coil 153 in response to W₁ and F₁ (A). The vertical dynamic focus generator 189 responds to a signal W₁ which is at a fixed level in the absence of writing in the tube 136 and to a R₁, E₁ and P₁ vertical deflection gate signal. The generator 188 responds to a W₁ vertical DF gate signal to provide vertical dynamic focusing during each vertical sweep of writing. A W₁ horizontal DF (deflection) timing signal is applied to the vertical dynamic focus generator 178 which applies a sweep signal through the OR gate 167 to the amplifier 174. During writing or recording, the video data is recorded in vertical lines each representing a range sweep with the horizontal position representing antenna azimuth position. During reading, the recorded data is read horizontally to simultaneously read a plurality of signals at the same range interval with the vertical sweep occurring in a relatively short period of time. As will be explained subsequently, reading is performed with a triangular or "jizzle" waveform in the horizontal dimension and a range sweep in the vertical dimension. To provide correction of a frequency modulation derived from the motion compensation, the same stored data is read twice from each tube with the triangular waveform.

For writing, a W₁ horizontal sweep timing signal is applied to a horizontal sweep generator 180 which applies a sweep signal through an OR gate 203 to a horizontal sweep amplifier 182 and to a horizontal deflection circuit 184 which in turn controls the horizontal sweep coil 154. Also for writing, a W₁ vertical sweep timing signal is applied to a write vertical sweep generator 192 which applies a vertical sweep signal through an OR gate 191 and a vertical sweep amplifier 179 to a vertical deflection circuit 194, which in turn controls the vertical deflection coil 152.

During reading, a W₁ vertical sweep timing signal and a R₁, E₁ and P₁ vertical sweep timing signal are applied to a read vertical sweep generator 193 which applies a vertical sweep signal to the OR gate 191 to control the vertical deflection coil 152. To control the horizontal position during reading, a horizontal sweep generator 200 responds to signals W₁ and F₂ (A) to develop the triangular or "jizzle" waveform which is applied to the OR gate 203. During prime and erase operations, the vertical sweep generator 193 controls the vertical deflection coil 152 and the generator 200 controls the horizontal sweep. During write, read, erase and prime, W₁, R₁, E₁ or P₁ timing signals are applied to a decelerating potential gate circuit 208 which energizes a collector and decelerating potential circuit 210 for applying required potentials to the decelerator screen 144 and to the collector 140. Also, potential switch circuits 214 respond to timing signals W₁, R₁, E₁ and P₁ to control a storage screen potential circuit 216 to apply the required potential to the storage screen 142. The read-out signal is applied from the collector 140 through a gated read amplifier 220 controlled by a read timing signal R₁ which applies the interrogated signal to a lead 222. The horizontal sweep generator 200 applies triangular signals to an AND gate 198 which responds to the R₁ signal to apply the signal to a lead 205 for controlling frequency correction during processing. The elements for controlling the storage tube 138 of FIG. 5 are similar to those of FIG. 4 and similar elements are designated with similar reference numerals except followed by the subscript a. The horizontal sweep generator 200a is coupled to an AND gate 199 responsive to the signal R₂ to apply the triangular signal to the lead 205.

Referring now to FIG. 6, the video signals read from the storage tubes 136 and 138 during each vertical read sweep (two consecutive vertical read sweeps at each tube) are applied to a variable width clutter filter 230 which may include a plurality of gated amplifiers such as 232, 234 and 236, each in a different parallel channel, which amplifiers respectively apply the signals to filters 238, 240 and 242. As shown by spectral diagrams 244 and 246, the passband may be selected to be progressively narrower (or the rejection band progressively wider) between filters 238 and 242 so that a wider spectral width of clutter is rejected. The gated amplifiers are controlled through leads such as 233 and 237 in response to the V_(LOS) signal which is applied to an analog to digital converter 248 to develop a five bit binary number, for example. The digital number is applied in parallel, for example, to a register 250 in response to timing signals S₁ or S₂. The register 250 may include five flip flops and be of a type well known in the art being responsive to a register clock pulse RC₁ or RC₂ to fill the register with new data at the center of each write cycle and prior to reading. A V_(LOS) signal for each selected sector of antenna azimuth scan is available to be utilized when the corresponding data is read from a storage tube. A decoder circuit 254 responds to the contents of the register 250 to develop a signal on one of 32 output leads coupled to a mask logic circuit 256. As is well known in the art, the mask circuit 256 may be a diode gating structure so that every other count selects a different lead such as 233 and 237 to apply an output gate signal to the selected gated amplifier. Thus, 16 leads may be coupled to different ones of 16 gated amplifiers to select a clutter filter having a width matched to the average clutter width of an azimuth sector of 280 radar sweeps or pulse transmissions in the illustrated system, or during approximately one-tenth of the total azimuth scan angle.

The output terminals of the filters such as 238, 240 and 242 are applied to an exclusive "OR" summing circuit 260 which passes a spectral region below the removed clutter signal through an amplifier 262 to a lead 264. A balanced mixer 266 responds to the video signal including the target signal to develop a frequency translation to return the target signal to its narrow spectral width. A correlation or swept oscillator 268 is coupled to the mixer 266 through a lead 269 and responds to a step attenuator 270 through a lead 267, which attenuator selects a frequency range for sweeping the target signal in response to the decoded rate of velocity change developed by the decoder 254 and applied to the step attenuator 270 through a composite lead 259 which may include 32 wires. For changing the phase of the frequency correction during the two periods of reading data from the storage tube 136 or 138, a paraphase amplifier 272 responds to the triangular horizontal read signal on the lead 205 to apply the signal to a phase selection circuit 274 which responds to a vertical read synchronizing pulse that occurs prior to each period of reading. The phase selected signal is applied from the phase selector 274 through a lead 275 to the step attenuator 270. It is to be noted that the vertical sweep gate signals for R₁ and R₂ (FIG. 8) may be utilized after passing through an OR gate (not shown) as the vertical synchronizing pulse.

The video signal with clutter removed and the target signal having its spectral width corrected during one reading period for improved signal to noise ratio, and translated to a greater spectral width during the other reading period of the same data, is applied from the mixer 266 through a lead 271 and in turn through an amplifier 280 to doppler filters such as 282 and 284 of a doppler filter bank 286. It is to be noted that the filter bank 286 is utilized for each range interval read from the tube so that only a relatively few filters such as 28 are required for doppler filtering during searching. The signals applied through the doppler filters such as 282 and 284 are then applied to respective detectors 288 and 291. Any conventional type of detectors may be utilized such as a mean level detector shown in U.S. Pat. No. 3,057,955, "Mean Level Detector" by Samuel Thaler. An OR gate 290 which may be of the inclusive "or" type, responds to the detected signal to apply the signal through a video threshold detector 292 and an AND gate 294 controlled by a sweep pulse for R₁, E₁ and P₁ of the waveform 698 (FIG. 26) or a sweep pulse for R₂, E₂ and P₂ combined in an OR gate (not shown). The detected signals are applied from the AND gate 294 to a range-azimuth display system 296. An AND gate 298 responds to an OR gate 299 in turn responsive to signals R₁ or R₂ to develop a range sweep for providing the vertical deflection of the beam in the display tube. Because the azimuth or η_(S) position is delayed, a register 304 receives the η_(S) signal from an A to D converter 303 and delays the η_(S) signal, from each azimuth sweep sector, before contnrolling the horizontal or azimuth beam position of the display tube. Signals S₁ or S₂ and RC₁ or RC₂ gate the digital data in parallel into the azimuth register 304. A digital to analog converter 307 may respond to the contents of the register 304 to apply a step voltage to provide azimuth or horizontal positioning of target signals such as 740 and 742.

Referring now to FIGS. 7, 8 and 9, the timing control system may include a clock 312 operating at 200 KC (kilocycles) in the illustrated system. It is to be understood that the illustrated timing frequency and intervals is only for ease of understanding and is not to limit the scope of the invention to any particular frequency or timing arrangement. Counters A, B, C and D of respective circuits 314, 309, 311 and 313, respectively including two, five, five and five flip flops, may be utilized in series for providing respective divisions of 4, 25, 20 and 22. As is well known in the art, each of the counters may include a gate responsive to a selected count such as the count of 21 for the D counter to energize a single shot circuit and apply a pulse to a reset gate at the end of the count 21. Also, the C counter may be arranged to include a binary counter and a decade counter. The counters may either use internal translation clocking or other clocking arrangement as are well known in the art. A "jizzle" or triangular logic circuit 315 responds to the counter A to develop DF gate signals F₁ (A) and SW gate signals F₂ (A) for controlling both storage tubes. A circuit 325 also responds to the counters C and D and to a PRF or synchronizing pulse to develop the signals W₁, R₁, E₁, P₁, W₁ Horiz. sw. gate or pulse, W₁ Vert. sw. gate for controlling the storage tube 136 and similar signals for the storage tube 138 as well as other signals as will be explained subsequently. Referring temporarily to the waveforms of FIGS. 26 and 27, the decoded states of the counters C and D are shown as D₀ to D₂₁, which states are continually repetitive. The W₁ write period of a waveform 690 occurs between D₀ and D₁₃, the R₁ period of a waveform 694 occurs during D₁₄ and D₁₅, the E₁ period of a waveform 770 occurs during D₁₆ and D₁₇ and the P₁ period of a waveform 771 between D₁₈ and D₂₁. The R₂ period of a waveform 696 occurs during D₃ and D₄ the E₂ period of a waveform 714 occurs during D₅ and D₆, the P₂ period of a waveform 716 occurs during D₇ to D₁₀ and the W₂ period of a waveform 692 occurs from D₁₁ through D₂. The W₁ horizontal sweep pulse of a waveform 717 occurs between a C₁ time after D₀ through D₁₃ and the W₂ horizontal sweep pulse of a waveform 772 occurs between a C₁ time after D₁₁ through D₂.

As shown in FIG. 8, set-reset type flip flops are coupled to the D terms to develop the W₁, W₁, R₁, E₁ and P₁ terms for the first tube as well as similar terms for the second tube. For developing the vertical sweep gate signal for R₁, E₁ and P₁ of a waveform 698, and AND gate 316 responds to the W₁ signal and a signal from the true terminal of a flip flop 318 which is respectively set and reset in response to terms C₁ and C₁₉. The vertical dynamic focus signal to R₁, E₁ and P₁ of a waveform 710 is developed by an AND gate 310 in response to W₁ and a signal from the true terminal of a flip flop 322 being respectively set and reset in response to terms C₁ and C₁₀. The vertical sweep pulse and the vertical deflection pulse for R₂, E₂ and P₂ are developed in a similar manner in response to AND gates 324 and 326 responding to W₂ as well as to the flip flop 318 and 322. A vertical sweep gate signal for R₁ and a vertical sweep gate signal for R₂ are developed in respective AND gates 317 and 319 for controlling the paraphase amplifier of FIG. 6. Gating signals S₁ and S₂ and clocking signals RC₁ and RC₂ are also developed by flip flops for the process control and the display, as will be subsequently explained in further detail.

Referring also to FIG. 9, the W₁ horizontal deflection timing pulse of a waveform 778 (FIG. 27) is developed by a flip flop 330 being set in response to an AND gate 332 and being reset in response to the term D₆. The AND gate 332 responds to C₁ and D₀ and to a PRF pulse of a waveform 775. The write horizontal sweep pulse of the waveform 717 is developed by a flip flop 334 being set in response to the AND gate 332 and reset in response to the term D₁₃. The W₁ vertical sweep gate signal of a waveform 776 and the W₁ vertical DF pulse of a waveform 777 are developed by respective single shot circuits 336 and 338 being triggered by an AND gate 340 in turn responsive to the flip flop 334 and the PRF pulse. An unblanking pulse of a waveform 705 for the tube 136 is developed by an OR gate 342 responsive to an AND gate 344 or to the vertical sweep gate pulse from the gate 316 (FIG. 8). The signals of the waveforms 776, 777 and 705 during W₁ time occur at each PRF pulse which may be at each 0.5 millisecond while the D periods may be of 10 milliseconds duration. The AND gate 344 responds to the W₁ horizontal sweep gate pulse of the waveform 717 and the W₁ vertical sweep gate pulse of the waveform 701. Similar gating pulses are developed for the second tube 138 as is shown in FIG. 9. It is to be noted that the W₁ horizontal DF pulse, the W₁ horizontal sweep pulse, the W₁ vertical sweep pulse, the W₁ vertical DF pulse and the unblanking pulse, as well as similar signals for the tube 138, are initiated at time C₁ after other voltage switching is completed at the storage tubes, before starting the deflection sweeps.

Referring now to the circuit diagram of FIG. 10, the phase rotator 84 utilized in the system of FIG. 3 to provide coherent operation responds at leads 81 and 83 to the quadrature voltage signals stored in the box car circuits 80 and 82 for each transmitted pulse. The frequency correction signal for velocity compensation is applied from the lead 109 to a transformer 350 as well as through a 90° phase shifter 352 to a transformer 354. The secondary windings of the transformers 350 and 354 which are center tapped to ground are respectively applied to diode rectifier bridges 356 and 360 to control current flow as a function of the amplitude of the signals on the lead 81 and 83 and are coupled to center taps of respective windings 362 and 364 of transformers 366 and 368. Secondary windings 370 and 372 are coupled between ground and a lead 374 to which the amplitude controlled signals are applied for summing in a summer 375. The summed signals are applied to the lead 85 as the phase shifted and frequency corrected signal of a form cos (wt + φ).

Referring now to FIGS. 11, 15 and 16, the linear predictor circuit 30 of FIG. 3 includes a space to aircraft converting circuit that responds to velocity vectors V_(I), V_(J) and V_(K) in horizon stabilized (earth oriented coordinates unresolved for compass heading) coordinates to develop velocity vectors V_(i), V_(j) and V_(k) in aircraft coordinates. As may be seen in FIG. 15, the V_(I) vector is horizontal with respect to the earth's surface, the V_(J) vector is at right angles to V_(I) and parallel to the surface of the earth and the V_(K) vector is vertical or at right angles to the surface of the earth. The pitch angle θ is the angle below the V_(I) vector and the roll angle φ is the antenna angle of the aircraft vector V_(j) in a vertical plane from the V_(J) vector. Inertial type stable platforms or doppler navigator systems having heading resolvers for developing these space or horizon stabilized reference vectors are well known in the art. As shown in the aircraft coordinate diagram of FIG. 16, the vector V_(i) is parallel to the longitudinal or thrust axis of the aircraft, V_(j) is parallel to the wings and at right angles to V_(i) and V_(k) is at right angles to V_(i) and V_(j). Lines 380 are the antenna line-of-sight or boresight axis and are at an azimuth angle η from the vector V_(i) and at an elevation or depression angle ξ below the V_(i) or axis. The elevation angle with respect to the horizontal is ξ _(S) and the azimuth angle in a horizontal plane parallel to the horizon is η_(S). If no pitch or roll is present, the η and ξ angles of FIG. 16 respectively correspond to η_(S) and ξ_(S). The resolving may be performed by any suitable resolver such as sine-cosine potentiometers which are nonlinear resistance devices developing an output voltage equal to the input voltage times the sine or cosine of the input angular shaft position. Resolver circuits of this type are well known in the art and will not be explained in further detail. It is to be noted that although in the illustrated system the values η and ξ are restrained to less than ± 90 degrees, the system may provide resolving systems over any desired number of angular degrees.

Referring now principally to FIG. 11, the vector signal V_(I) is applied to a θ resolver 390 which applies a V_(I) cos θ term to a summer 392 and a V_(i) sin θ to a φ resolver 394. Terms V_(I) sin θ cos φ and V_(I) sin θ sin φ are applied from the resolver 394 to respective summers 396 and 398. The V_(J) signal is applied to a φ resolver 400 which in turn applies a term V_(J) cos φ to the summer 398 and a term -V_(J) sin φ to the summer 396. The signal V_(K) is applied to a θ resolver 402 which in turn applies a term V_(K) cos θ to a resolver 404 and a term -V_(K) sin θ to the summer 392. The φ resolver 404 applies a term V_(K) cos θ cos φ to the summer 396 and a term V_(K) cos θ sin φ to the summer 398. The summers 392, 398 and 396 respectively develop the signals V_(i), V_(j) and V_(k) in aircraft coordinates. The conversion circuit of FIG. 11 may be expressed by the following equations:

    V.sub.i = V.sub.I cos θ - V.sub.K sin θ

    V.sub.j = V.sub.J cos φ + V.sub.K cos θ sin φ + V.sub.I sin θ sin φ

    V.sub.k = V.sub.K cos θ cos φ + V.sub.I sin θ cos φ - V.sub.J sin φ.

Referring now to FIG. 12, the rate of change of velocity along the antenna line-of-sight due to antenna scan, or the partial derivative of velocity with η_(S), may be developed in the process control circuit 42 in response to the velocity vectors in aircraft coordinates. A θ resolver 391 responds to V_(i) to apply a term V_(i) cos θ to a φ resolver 393 which applies a term V_(i) cos φ cos θ to a summer 395 and a term -V_(i) sin φ cos θ to a summer 397. A θ resolver 399 responds to V_(j) to apply a -V_(j) cos θ term to a φ resolver 401 which in turn applies a term -V_(j) cos φ cos θ to a summer 403. The resolver 399 also applies a term -V_(j) sin θ to the summer 397. A θ resolver 405 responds to the V_(k) signal to apply a term V_(k) cos θ to a φ resolver 407 which applies a term V_(k) sin φ cos θ to the summer 403. The resolver 405 also applies a term V_(k) sin θ to the summer 395. The summers 397, 395 and 403 develop the respective velocity rate vector terms V_(i), V_(j) and V_(k) in aircraft coordinates. The following equations also express the operation of the circuit of FIG. 12.

    -V.sub.i = V.sub.k sin φ cos θ - V.sub.j cos φ cos θ

    -V.sub.j = V.sub.k sin θ + V.sub.i cos φ cos θ

    -V.sub.k = V.sub.j sin θ - V.sub.i sin φ cos θ

Referring now to FIG. 13, the line-of-sight velocity signal V_(LOS) is developed by a circuit 110 in response to the signals V_(i) V_(j) and V_(k) in aircraft coordinates. An η resolver 410 responds to the signal V_(i) to apply a term V_(i) cos η to a summer 412. An η resolver 414 responds to the signal V_(j) to apply a term V_(j) sin η to the summer 412. The summer 412 applies a signal to an ξ resolver 416 which applies a term V_(i) cos η cos ξ + V_(j) sin η cos ξ to a summer 418. An ξ resolver 420 responds to the signal V_(k) to apply a term V_(k) sin ξ to the summer 418 which in turn develops the signal V_(LOS) as a function of the antenna angles η and ξ and the aircraft velocity vectors. The equation for developing V_(LOS) is:

    V.sub.LOS = V.sub.i  cos η cos ξ + V.sub.j sin η cos ξ - V.sub.k sin ξ

Referring now to FIG. 14, the process control signal V_(LOS) is developed by a circuit 116 including resolver structure similar to that of FIG. 13 except the terms V_(i), V_(j) and V_(k) are utilized as developed by the circuit of FIG. 12. The resolvers and summers of FIG. 14 are similar to those of FIG. 13 and have similar reference numerals followed by the subscript a. It is to be noted that the resolvers and summers of FIGS. 13 and 14 are shown with more detail than those of FIGS. 11 and 12 to illustrate a typical arrangement.

Referring now to FIG. 17, the paraphase amplifier 272 and the phase selection circuit 274 will be explained in further detail. To reverse the phase of the signal correction signal during each sequential one of two read rasters for each storage tube (each storage surface of data being read twice) a flip flop 440 responds to a vertical sweep pulse for R₁ or a vertical sweep pulse for R₂ developed by AND gates 317 and 319 of FIG. 8 to be alternately set and reset and apply a high or a low voltage to a lead 442. The flip flop 440 is of a type well known in the art and will not be explained in further detail.

When the signal on the lead 442 is at a low or approximately ground level, a diode 444 is biased into conduction and an npn type transistor 446 is biased out of conduction to apply a positive potential to a lead 448 which biases a pnp type transistor 472 out of conduction. A zener diode 445 is coupled between the base of the transistor 446 and the anode of the diode 444. Diode bridge circuits 450 and 452 respectively include diodes 451, 453, 463 and 465 and diodes 455, 457, 459 and 461. The cathode and anode of respective diodes 463 and 451 are coupled through a lead 454 to the emitter of an npn type transistor 458 and the cathode and anode of respective diodes 455 and 459 are coupled through a lead 456 to the collector of the transistor 458 of the paraphase amplifier circuit 272. The base of the transistor 458 is coupled to the lead 205 to respond to the triangular or "jizzle" signal and the collector and emitter are coupled to suitable sources of biasing potential. The collector of the transistor 446 is coupled through resistors 462 and 464 to a second terminal of the bridge circuit 450 at the anodes of the diodes 463 and 465. A -22 volt terminal 468 is coupled through a resistor 470 to a point between the resistors 462 and 464 as well as to a second terminal of the bridge circuit 452 at the cathodes of the diodes 459 and 461. The transistor 472 has a base coupled through a zener diode 474 and a diode 476 to the lead 448, an emitter coupled to ground and a collector coupled through a suitable resistor to a -25 volt terminal 475 as well as through resistors 477 and 478 to a third terminal of the bridge circuit 452 at the anodes of diodes 455 and 457. A +22 volt terminal 480 is coupled through a resistor 482 to a point between the resistors 477 and 478 which in turn is coupled to the cathodes of the diodes 451 and 453. The other terminals of the bridges 450 and 452 respectively at the cathode and anodes of diodes 457 and 461 and at the cathode and anode of diodes 465 and 453 are coupled to a lead 490 and through an amplifier 492 to the lead 275 which is in turn coupled to the step attenuator 270 of FIG. 6.

In operation, when a low voltage is applied to the lead 442, the transistor 446 is biased out of conduction and a positive potential is applied to the resistor 462 to increase the conduction of the diodes 463 and 465 of the bridge circuit 450 and to bias the transistor 472 out of conduction. Also, the diodes 459 and 461 are biased out of conduction. The -25 volts is applied from the terminal 475 to bias the diodes 455 and 457 of the bridge circuit 452 out of conduction. Also, the diodes 451 and 453 of the bridge 450 are biased into conduction. The signals on the leads 454 and 456 are 180° out of phase. The ratio of the conductance of the diode bridges determines the phase of the triangular signal applied to the lead 490, changing 180° in response to the flip flop 440. When the voltage is at a high level on the lead 442, the transistors 446 and 472 are biased into conduction to bias the diodes of the bridge circuit 450 out of conduction and the diodes of the bridge circuit 452 into conduction to shift the phase of the signal on the lead 490 by 180°. The phase of the signal on the lead 490 changes at each vertical synchronizing pulse during reading from a storage tube so that two frequency translations are provided of the target signal, one developing the desired narrow spectrum.

Referring now to the step attenuator circuit 270 of FIG. 18, the triangular signal alternately changing phase during each of two readings of the data on the storage screen of a tube is applied from the lead 275 through suitable biasing elements to the base of an npn type transistor 510 having a collector coupled through an inductor 511 and a resistor 512 to a suitable source of potential such as a +22 volt terminal 514. The collector of the transistor 510 is also coupled through a coupling capacitor to the base of an npn type transistor 516 having a collector coupled to the terminal 514 and an emitter coupled through a resistor 518 to ground. The emitter of the transistor 516 is also coupled through a suitable coupling capacitor to the lead 267 on which a triangular waveform of selected slope and phase controls the frequency sweeping range of the correlation oscillator 268 of FIG. 6. The amplitude of the signal on the lead 267 is controlled at the emitter of the transistor 510 which is coupled through an inductor 520 and a resistor 522 to ground for providing a high AC (alternating current) impedance to ground. A field effect transistor 524 has a first electrode coupled through a capacitor 526 to the emitter of the transistor 510 and a second electrode coupled to ground. The control terminal of the transistor 524 is coupled through a capacitor 528 to the capacitor 526 as well as through a resistor 530 to a -10 volt terminal 532. The field effect transistor 524 may be any conventional type such as a 2N2498, as is well known in the art. The control terminal of the transistor 524 is also coupled through a resistor 534 to the collector of a pnp type transistor 538 having a base coupled to a +10 volt terminal 540 and an emitter coupled through a dividing resistor 542 to the terminal 514.

The emitter of the transistor 538 is also coupled to a lead 544 which in turn is coupled through a plurality of resistors such as 546, 548, 550 and 552 to the anodes of respective diodes 554, 556, 558 and 560. The cathode of each of the diodes such as 554 and 556 is coupled to a lead of the composite lead 259 from the decoder 254 (FIG. 6) on which a signal which may be normally at 12 volts is switched to 0 volts on a selected line as determined by the digital value of the decoded line-of-sight velocity rate. The values of the resistors increase from the resistor 552 to the resistor 546 so that ground potential applied to a diode connected to an increased resistor value develops an increased value of voltage on the lead 544 and an increased current flowing into the emitter of the transistor 538. An increase of voltage on the lead 544 increases the impedance through the transistor 524 to increase the gain of the transistor 510 and increase the amplitude of the signal on the lead 267. Thus the frequency excursion of the correlation oscillator 268 increases with a voltage rise on the lead 544 so that the diodes 554 and 556 are connected to respond to the higher values of V_(LOS). It is to be again noted that the triangular waveform on the lead 494 changes phase during each vertical read sweep. The digital value of V_(LOS) is changed each write period so that a new value is utilized from each scan sector defined by the number of vertical lines recorded on each tube.

Referring now to FIG. 19, the correlation oscillator 268 may include a paraphase amplifier 570 responsive to the triangular waveform on the lead 267 of a waveform 572 to develop signals (180° out of phase) of waveforms 574 and 576 on leads 578 and 580, which signals are applied to first and second voltage controlled oscillators 582 and 584. A mixer 586 or cos φ phase detector responds to the signals developed by the oscillators 582 and 584 to apply a signal through an amplifier 591, a clipper circuit 593 and an amplifier 595 to the lead 269 for controlling the swept frequency of the balanced mixer 266 of FIG. 6. The oscillators 582 and 584 may be selected to operate with respective center frequencies of 140 MC (megacycles) and 173.6 MC each with a frequency range of ± 4.4 MC as an illustration of their operation. The mixed signal may be centered at 33.6 MC with ± 11.2 MC variation to provide a correlation oscillator with a relatively wide dynamic range. It is to be noted that other types of voltage controlled oscillator systems as well known in the art may be utilized for the correlation oscillator 268 in the illustrated system.

Referring now to FIG. 20, a circuit that may be utilized for the balanced mixer 266 of FIG. 6 includes a mixing capacitor 586 coupled between the lead 264 and a resistor 588 in turn coupled to a voltage null point 589 which is coupled to resistors 590 and 592 and to a tap 594 of a resistor 596. The resistors 590 and 596 are coupled to the anode of a diode 598 and the resistor 592 and the other end of the resistor 596 are coupled to the cathode of a diode 600. Capacitors 602 and 604 are each coupled between ground and the anode of the diode 598 and capacitors 606 and 608 are each coupled between ground and the cathode of the diode 600. A transformer 612 includes a winding 614 having a center tap coupled to ground and the first and second ends respectively coupled to the cathode of the diode 598 and to the anode of the diode 600 for applying signals thereto 180 degrees out of phase from each other. The signal on the lead 269 is applied through a coupling capacitor 616 to a first end of a winding 618 of the transformer 612, the other end of the winding being coupled to ground. The point 589 is coupled through suitable coupling capacitors to the lead 271 where the alternately frequency corrected and uncorrected target signals are applied to the doppler filter bank 286 of FIG. 6. The mixer of FIG. 20 operates in a conventional manner with the diodes 598 and 600 to balance out and cancel, at the null point 589, components of the signal on the lead 269 and apply the heterodyned signal to the lead 271.

Referring now to FIG. 3, the operation of the system in accordance with the invention will be explained in further detail. The moving target detection system may operate with a noncoherent radar system to provide improved moving target detection capabilities while retaining the advantages of a relatively simple and high power transmitter such as a magnetron transmitter. Also, it is to be understood that the principles of the detecting system of the invention are applicable to other types of coherent radar transmitters and receivers as well known in the art. Although the illustrated system operates in a highly desirable manner at low or medium PRF, it is to be also understood that the system is operable over all ranges of pulse repetition frequency. Although the illustrated system is especially desirable during searching operations such as during azimuth or elevation scanning, it is to be noted that the principles of the invention are also applicable to tracking and to other types of scanning patterns. The magnetron transmitter in response to a PRF synchronizing pulse, transmits pulses of energy at the frequency f_(x) through the duplexer 60 into space from the antenna 62 which intercepts reflected energy from targets and from clutter such as main lobe ground clutter and side lobe ground clutter. The range gate 92 may effectively eliminate the side lobe clutter signals. The received RF (radio frequency) energy is at a frequency of f_(x) + f_(ds) where f_(ds) is the doppler shift caused by the movement of the target and the movement, relative to the radar illuminated patch or portion of the earth's surface, of the platform on which the radar is mounted such as an aircraft. The RF signal at the frequency f_(x) + f_(ds) and a signal at a frequency f_(x) + f_(o) from the stable local oscillator 70 are applied to the mixer 88 to apply the signal at the frequency f_(o) - f_(ds) to the mixer 86. The mixer 68 responds to the signal from the local oscillator 70 and a sample signal from the magnetron transmitter to apply an IF frequency signal, at a phase determined by the phase of the transmitted signal f_(x), to the phase detectors 74 and 78. The local oscillator 70 functions as the phase reference at RF frequencies. The stable IF reference oscillator 72 at the frequency f_(o) allows the phase detectors 74 and 78 to accurately detect the phase of the transmitted pulse after conversion to IF frequency. In response to the PRF synchronizing pulse, voltages representing the quadrature phase relations are stored in the box car circuits 80 and 82 during each interpulse period while energy is being returned from objects in space and on the ground.

For converting the IF signal to a coherent signal, the phase correction is applied to the signal at the frequency f_(o) - f_(ds) to develop the velocity compensated mixed frequency signal on the lead 90 as determined by the VCO 108. Referring also to the phase rotator of FIG. 10, the voltage applied to the center taps of the primary windings 362 and 364 determines the phase position of the summed signal on the lead 85 at any frequency applied to the lead 109. The signals on the leads 81 and 83 are proportional to the cosine and sine of the phase angle of the transmitted energy after conversion to IF frequency and vary the diode balance or conductance of the linear diodes to apply the vector signals to the lead 374. Thus the signal is mixed at the mixer 86 to develop a coherent signal, that is, a signal that is always in phase with the reference oscillator 72 so that signals developed from return energy of different sequential transmitted pulses have a known phase relation.

The coherent signal on the lead 90 may have a frequency f₁ - Δ S where Δ S is the relatively small loop correction frequency. The velocity error caused by platform or aircraft motion relative to the patch of the earth's surface illuminated by the main lobe antenna pattern is eliminated by shifting the central line of the clutter spectrum to a selected frequency f₁. As is well known in the art, the clutter spectrum of signals repeating at multiples of the PRF may vary over a wide range of frequencies as a function of the platform velocity. As shown in FIGS. 21 and 22, clutter spectral signals may shift in frequency as the platform changes its position relative to the main lobe clutter reflectors which may be a varying ground configuration. The aircraft or platform 599 containing the system in accordance with the invention is flying along a flight path 603 relative to a target 605 which may be considered stationary for a short interval of time and relative to a range gated terrain surface 607. In response to three transmitted pulses, three sequential main lobe patterns (1), (2) and (3) may be formed. Although the target is illuminated during all three time periods, different areas of the terrain 607 are illuminated and the clutter shifts in doppler frequency (not considering the azimuth scanning motion of the antenna). Thus a clutter spectrum 609 of a spectral diagram 611, which may occur in the region of frequency f_(o) ' or f_(o) - f_(ds), includes clutter returns from lobes (1), (2) and (3) and extends over a relatively wide range of frequency because of the change in clutter doppler frequency. Target signals T₁ of repetitive spectral lines 682 and 684 and target signals T₂ of repetitive spectral lines 683 and 685 have a relatively narrow spectral width because of the relatively small change in doppler frequency. The clutter spectrum 609 also repeats at the signal 687a. The velocity compensation system of the invention translates the clutter signals 609 and 687a in frequency so that the central clutter spectral signal 609 is centered when heterodyned to IF frequency at the frequency f₁ as a signal 687b of a spectral diagram 797 representing stationary clutter or effectively representing stationary clutter. The filter 94 is selected with a narrow band centered at the frequency f₁ and the discriminator 98 is centered at frequency f₁ to maintain the frequency of signal 687b thereat.

As shown in FIG. 23, the clutter filter 94 has a narrow passband 668 and the discriminator 98 has a voltage versus frequency characteristic indicated by a curve 670. It is to be noted that the velocity compensating loop functions to always maintain the central line of the clutter spectrum at the frequency f₁, the spectral signal thus being insensitive to the PRF. The loop 95 operates only when the clutter signal has sufficient amplitude to activate the signal activated gate 67. The linear predictor operation when the voltages are established such as by controlling the switch 100, assures that the closed loop 95 will track the mean frequency of the central line or the signal 671. If the central line were not tracked, the loop 95 would be required to operate over a relatively wide dynamic range. The clutter spectral signal 671 is then passed through the function circuit 102 which may be essentially a low pass filter. The closed loop correction signal is then applied to the summer 104 and would provide adequate velocity correction except that the clutter signals vary in amplitude and may be temporarily absent when flying over terrain of varying configurations. Thus the closed loop velocity compensation is utilized for second order corrections in the illustrated system and the linear predictor 30 predicts clutter doppler shift for first order corrections as a function of the systematic phase variations due to antenna line-of-sight velocity to stationary clutter reflectors. In the absence of clutter, the loop 95 is inactive in response to the signal actuated gate 67 and the open loop linear predictor maintains the clutter spectrum at the proper frequencies. The velocity vectors V_(I), V_(J) and V_(K) in space coordinates may be derived from the doppler or inertial navigation system 32. Also, the velocity vectors in space coordinates may be derived from a stable platform and a computer or from systems utilizing analog or digital techniques to compute the velocity vectors. As explained relative to FIGS. 11 and 13, the line-of-sight velocity signal V_(LOS) may be developed from resolvers responding to the velocity vectors in aircraft coordinates and to the antenna angles η and ξ relative to the aircraft. The summer 104 thus receives a continuous predicted correction signal for controlling the VCO 108 which operates at a frequency of f_(o) - f₁ - f_(ds) + Δ S. The signal on the lead 109 at this frequency is applied to the mixer 86 (after a phase correction) to maintain the velocity compensated signal on the lead 90 with the clutter spectrum signal 687 (FIG. 22) centered at frequency f₁. It is to be noted that if other types of coherent transmitters and receivers are utilized, the lead 109 may be directly coupled to the mixer 86 within the scope of the invention. As the clutter signal is translated to the frequency f₁, the target signals are translated over a relatively wide frequency range as shown by spectral signals 689 and 681 for target T₁ and by spectral signals 693 and 695 for target T₂. The signal designations (1), (2) and (3) of the signal 689 correspond to the positions of the lobes in FIG. 21.

The signal on the lead 90 is mixed in the mixer 120 to a video frequency in response to the balanced modulator or mixer 126 in turn responsive to the VCO 124 operating at the frequency f₁. The mixer 120 functions as a synchronous detector in response to the reference frequency of the VCO 124 offset by a frequency of one-half PRF to provide spectral folding at one-half PRF of the signal on the lead 90. For accurately tracking the frequency of the signal, the phase detector 130 responds to the signal on the lead 96 and the signal developed by the VCO 124 to apply a control signal through the function circuit 132 which operates as a low pass filter and amplifier to maintain the synchronous frequency f₁ at the output of the VCO 124, that is, the VCO 124 is locked in phase with the signal on the lead 90. As the VCO 124 is centered at the frequency f₁, the synchronous detection operation continues in the absence of clutter signals on the lead 96 such as when the gate 67 is open. The balanced modulator 126 may be a conventional circuit including a transformer with the signal from the VCO 124 applied to the primary winding and the alternately high and low signal applied from the flip flop 127 to the center tap of the secondary winding. Oppositely poled diodes are connected between the ends of the secondary winding to an output lead and a parallel inductor and capacitor to ground. The modulator 126 provides a frequency offset (either a decrease or an increase) to the signal applied to the mixer 120. As a result of changing the modulation with each PRF synchronizing pulse, the clutter signals 687 and 697 of FIG. 22 are translated to a frequency of 1/2 PRF when the spectrum folds as shown by a clutter signal 699 of a spectral diagram 703. The spectrum is folded in the mixer 120 at a folding line of frequency f₁ + 1/2 PRF of the diagram 797. As frequency is equal to the derivative of phase angle with respect to time, a frequency or PRF of 2 KC would develop a 360 degree phase change in a period of 1/2000 seconds. If the frequency is 1 KC, the signal shifts in phase by 180° during a time interval of 1/2000 seconds. As 1 KC is equal to 1/2 PRF and if the phase is changed by 180° in a PRF interval, the signal such as the clutter signal is frequency translated 1/2 PRF as the spectrum of the diagram 797 folds at a 1/2 PRF point. Because the VCO 124 develops a reference signal that is synchronous in phase with the signal on the lead 90, the video signal is folded to DC. Thus, in the illustrated system, 1/2 PRF corresponds to a 180° phase change or phase reversal in a PRF time interval by the flip flop 127 which causes a 1/2 PRF offset (increase or decrease in frequency) of the signal developed by the modulator 126 and of the clutter signal because the VCO 124 is maintained phase locked at the frequency f₁ (or f₁ - Δ S) of the clutter signal on the lead 90. Also, other PRF frequency offsets in the modulator 126 may be provided in accordance with the principles of the invention such as a 1/4 PRF offset (with folding to DC of the spectral diagram 797 at a 1/4 PRF point) by changing the phase 90° every PRF or frequency interval at the modulator 126 or a 1/8 PRF offset (with folding to DC of the spectral diagram 797 at a 1/8 PRF point) by changing the phase 45 degrees every PRF or frequency interval at the modulator 126. Conventional counter type circuits may be utilized in place of the flip flop 127 for PRF offsets such as 1/4 and 1/8 as will be explained relative to FIG. 29. As will be explained subsequently, the frequency of the input signal (f₁) may vary from the frequency f₁ of the oscillator 124 in accordance with the principles of the invention.

The target signals 701 and 702 are thus established in a spectral region close to DC so as to be well within the frequency limitations of storage devices such as the storage tubes 136 and 138. The target T₁ of the spectral signal 701, which is derived from the spectral signal 689, is folded into the interval from DC to 1/2 PRF and the target T₂ of a signal 702 is frequency shifted while in the 1/2 PRF interval. The dotted target signal T₂ is shown to indicate that a target signal in any position will appear in the 1/2 PRF interval of the spectral diagram 703. It is to be noted that the direction of frequency change for an increasing doppler frequency is reversed when the target T₁ is folded into the 1/2 PRF interval as indicated by the direction of the arrows. By establishing the clutter signal at a fixed frequency, any target signal may be detected within the DC to one-half PRF range of the folded spectrum.

Referring to FIG. 24, a curve 807 shows the percent response of a typical storage tube to the frequency of the applied signals. For example, in the illustrated system, the 1/2 PRF frequency may be at F_(S) spatial cycles and in a region of relatively low tube response. Thus, the central line of the spectral clutter signal centered at 1/2 PRF is attenuated in the storage tube without affecting the target signal because of the 1/2 PRF modulation in accordance with the invention. Similarly signals at frequencies larger than 1/2 PRF are substantially rejected in the tube storage operation. It is to be noted that the system may operate with other types of storage devices or without storage and the principles of the invention are equally applicable thereto.

Referring now to FIGs. 4, 5, 25, 26 and 27, the clutter referenced video signal is applied on the lead 122 to the write amplifier and bias circuits 164 and 164a at the storage tubes 136 and 138. As may be seen in FIG. 26, writing is performed during W₁ horizontal sweep and W₂ horizontal sweep times on respective tubes 136 and 138 in response to pulses of the waveforms 717 and 773. The write horizontal sweep generator 180 responds to the W₁ horizontal sweep gate signal of the waveform 717 to apply a horizontal sweep signal to the horizontal deflection circuit 184 which controls the coil 154. The W₁ period of the waveform 690 may have a duration of 140 ms (milliseconds), for example, in the illustrated system. Also in the illustrated system the R₁, E₁ and P₁ pulses of respective waveforms 694, 770 and 771 may have respective durations of 20 ms, 20 ms and 40 ms. During the horizontal sweep, the signal from each range sweep is recorded in a different vertical line on the storage screen 142 as shown by a diagram 773 of FIg. 25. The horizontal parameter of the diagrams 774 and 773 may be η_(s) (Δ T) where η_(s) is δη_(s) /δ_(t) and Δ T is the time for scanning one antenna scan sector (frame) in azimuth. The write vertical sweep generator 192 responds to the W₁ vertical sweep gate signal of the waveform 776 developed by the single shot circuit 336 of FIG. 9 to control the vertical deflection circuit 194 and define the write vertical sweep during each range sweep or PRF interval. Pulses of the waveform 776 are developed in response to PRF pulses of a waveform 775 during W₁ time, the pulses having a duration of 370 microseconds, for example, when each D period is 10 millliseconds. For example, 280 vertical sweep signals may be developed by the generator 192 for recording data over a selected azimuth scan sector of approximately 12.6° when the antenna scans at a rate of 90° per second. It is to be noted that during the portion of the W₁ and W₂ periods which overlap in time, the same data is recorded in both storage tubes. Thus overlap in time results in improved detection of a moving target that may be split or divided, that is, positioned at the edge of a scan sector as defined by the last portion of the write raster of one tube. Sweep pulse generators that develop a sweep during each of a plurality of gated time intervals are well known in the art. During writing after the tube has been erased and primed, the dynamic focus amplifier 174 responds to the horizontal dynamic focus generator 178, which generator is controlled by the W₁ horizontal DF gate signal of the waveform 778 developed by the flip flop 330. Also, the write vertical deflection generator 188 responds to the W₁ vertical deflection gate signal of the waveform 777 developed by the single shot circuit 338 (FIG. 9) to apply a vertical focus signal to the amplifier 186. The W₁ vertical DF gate signal of the waveform 777 occurs during the W₁ period in response to each PRF pulse and each DF pulse may have a duration of 185 microseconds when the D period is 10 milliseconds, for example. The horizontal and vertical focus signals as well as the static focus voltage from the supply 172, provide a static and dynamic focus for magnetically controlling the beam, as is well known in the art.

Also during writing, the coding circuit 168 responds to the W₁ signal to control and establish the bias potential of the amplifier 164 prior to generation of the sweep voltages. The unblanking pulse of the waveform 705 also controls the coding circuit 168 during beam flyback times while writing as well as during reading, erasing and prime operations. The decelerator potential gates 208 respond to the signal W₁ to gate the required potential to the decelerator grid 144 prior to writing into the tube 136. Before completion of writing into the tube 136, the W₂ pulse of the waveform 692 and the W₂ horizontal sweep pulse of the waveform 772 are developed to perform a similar writing operation on the tube 138, the same data being recorded on both tubes during times D₁₁, D₁₂ and D₁₃.

At the completion of writing on tube 136, the reading operation is performed on that tube in response to an R₁ pulse of a waveform 694. It is to be noted that during writing in tube 136, information is being read from the tube 138 in response to the read pulse R₂ of a waveform 696. Reading is performed by the triangular or "jizzle" signal of the waveform 700 (FIG. 28) controlling the horizontal deflection and a vertical sweep for R₁, E₁ and P₁ signal of the waveform 698 controlling the vertical deflection in the range dimension as shown by a screen surface 774 in FIG. 25. Thus signals are read from each range interval over the data from a plurality of range sweeps of an azimuth sector to provide highly reliable target signal indication. Reading the stored data twice may be performed in 20 milliseconds in the illustrated system. The vertical sweep generator 193 responds to an inverted write signal W₁ and to the R₁, E₁ and P₁ vertical sweep gate signal of the waveform 698 of FIG. 26 to develop a vertical deflection sweep for 9 milliseconds in the illustrated system. The horizontal sweep generator 200 responds to W₁ and to the triangular sweep gate signal F₂ (A) of a waveform 727 of FIG. 28 to develop the triangular signal of a waveform 700 changing slope each 10 microseconds, for example. It is to be noted that the generator 200 may include an integrator being alternately charged and discharged in response to the pulses F₂ (A) to form the triangular sweep signals. The signal F₂ (A) may be formed at the output of the A counter 314 of FIG. 7. Thus, the tube is alternately swept in opposite directions throughout all range intervals as indicated by the surface 774 of FIG. 25. During R₁ time as shown by the waveform 694, the tube is swept twice in the vertical direction as controlled by the R₁, E₁ and P₁ vertical sweep signal of the waveform 698. During reading, the decelerating potential gates 208 control the decelerator potential and the potential switches 214 control the storage screen potential. The horizontal sweep generator 201 controls the horizontal dynamic focus in response to the signal F₁ (A) of a waveform 712 as developed in the A counter 314 of FIG. 7. The R₁ pulse of the waveform 694 is applied to the gated read amplifier 220 to pass the modulation sensed on the collector 140 during reading, which signal is applied to the lead 222 with the clutter signal 699 of FIG. 22 centered at 10 MC in the illustrated system, which is equivalent to 1/2 PRF after frequency conversion in the storage tubes.

After the reading operation in the tube 136 the erase operation is performed in response to an E₁ erase pulse of a waveform 770, and the vertical sweep signal for R₁, E₁ and P₁ of the waveform 698. Erasing is performed by writing a DC signal into the tube to normalize the storage element at full modulation level. The E₁ pulse is applied to the coding circuit 168 and a sufficiently high bias potential is applied to the intensity grid 148. The horizontal sweep generator 200 responds to the W₁ pulse or the absence of writing and to the signal F₂ (A) of the waveform 727 (FIG. 28) to apply the "jizzle" or triangular voltage of the waveform 700 to the horizontal deflection coil 154 during the erase period. At the same time, the vertical deflection generator 193 responds to the signal W₁ and to the R₁, E₁ and P₁ vertical deflection signal of the waveform 698 (FIG. 26) for applying a vertical sweep signal to the vertical deflection coil 152. Focusing during the erase period is controlled by the vertical deflection generator 189 responding to the signal W₁ and the R₁, E₁ and P₁ vertical deflection signal of a waveform 710 (FIG. 26). The focus is also controlled by the horizontal sweep generator 201 responding to W₁ and to the signal F₁ (A) of a waveform 712 (FIG. 28). The decelerating potential gates 208 and the potential switches 214 are controlled by the E₁ pulse to apply the required bias to the respective collector and storage screens just prior to starting of the sweep voltages.

Priming is accomplished by uniformly charging the storage surface by reducing the screen voltage below the critical potential and scanning the raster during a P₁ pulse of the waveform 708. The coding circuit 168 is controlled by the pulse P₁ and the decelerator potential gates 208 and the potential switches 214 are also gated in response to the pulse P₁. The horizontal sweep generator 200 responds to the signals W₁ and F₂ (A) to control the horizontal deflection circuit 184 to apply a horizontal sweep signal of the waveform 700 (FIG. 28) to the coil 154. The vertical sweep generator 193 responds to the signal W₁ and to the R₁, E₁ and P₁ vertical sweep gate of the waveform 698 to control the vertical deflection coil 152. The vertical dynamic focus generator 189 responds to W₁ and the vertical deflection signal of the waveform 710 to apply a vertical dynamic focus signal to the amplifier 186. The horizontal dynamic focus sweep generator 201 responds to W₁ and the signal F₁ (A) to apply a horizontal dynamic focus signal to the amplifier 174. After completion of a prime operation, the signal W₁ is repeated to control writing on the tube 136. The operation of the tube 138 of FIG. 5 is similar to that described for FIG. 4, occurring during times defined by the W₂ pulse of the waveform 692, the R₂ pulse of the waveform 696, the E₂ pulse of a waveform 714 and a P₂ pulse of a waveform 716 as well as vertical sweep gate pulses of a waveform 687c.

The signals read from the tubes 136 and 138 are applied from the leads 222 and 222a to the lead 231 of FIG. 6 and to the plurality of channels of the clutter filter 230, one of which is energized during each R₁ or R₂ pulse. The V_(LOS) signal which is the partial of V_(g) over the partial of azimuth angle η_(s) is continually developed and applied through the lead 117 to the A to D converter 248 to the register 250 in response to signals S₁ or S₂ of respective waveforms 741 and 743. It is to be noted that the signal V_(LOS) is also a function of the elevation angle ξ_(s) which is a constant in the illustrated azimith scan system. A clock pulse RC₁ or RC₂ of respective waveforms 747 and 749 may also be applied to the flip flops of the register 250. The signals S₁ and S₂ occur substantially at the center of the W₁ or W₂ write periods so a sample is available during reading. The V_(LOS) information is thus sampled at the center of the time period of each azimuth scan sector although either more frequent or less frequent sample periods may be utilized within the scope of the invention. The decoded signals are passed through the mask logic circuit 256 to apply a pulse to one of the 16 leads such as 233 to energize a gate such as 232. Each of the filters 238 to 242 has a passband characteristic that becomes progressively smaller by filter parameter selection as is well known in the art. The clutter signal has the narrowest width when the antenna is pointing along the ground velocity vector of the aircraft and becomes increasingly wide in frequency as the scan angle η_(s) increases. At a constant velocity, as the value of V_(LOS) increases from scan sector to scan sector, filters progressing from 242 to 238 are selected and as the angle η_(s) increases, filters 238 to 242 are progressively selected. Both positive and negative valves of M_(s) may provide the same values in the illustrated system. Thus the digital value of V_(LOS) is stored when the video signal is stored in one tube and when the data is read from that tube the stored V_(LOS) is utilized to energize a desired clutter filter. This operation provides complete elimination of the clutter signal while passing the maximum spectral width between DC and 1/2 PRF for detection of a target signal. The clutter signal 699 of FIG. 22 is removed in the filter such as 238 to pass a target signal 777 of a spectral diagram 779 to the lead 264. The percent response at 1/2 PRF as shown by the curve 807 (FIG. 24) is relatively low in the region of the clutter signal of the diagram 703 of FIG. 22. Thus signals at clutter frequencies and higher than 1/2 PRF in the diagram 779 are greatly attenuated by the response of the tubes. The target signal is maintained in the region of maximum frequency response in the system in accordance with the invention.

After removal of the clutter spectrum, the video signal is applied to the balanced mixer 266 for correcting the target spectral signal. As shown in FIG. 22, the target signals such as 701 is frequency shifted over a relatively wide spectral region as a result of the velocity compensation and would be unreliably detected because of the low output signal to RMS (root mean square) noise ratio. The shifted target signals of the diagram 703 may have a width of 500 cycles per second, for example. The target signal is thus translated in frequency an amount corresponding to the motion compensation of the corresponding signal applied to the mixer 266 at each instant of time. When the data is transferred to the register 250 during each writing interval in response to signals S₁ or S₂, the desired impedance is selected in the step attenuator circuit 270. In response to the triangular signal of the waveform 700 (FIG. 28), the correlation oscillator 268 is swept first with one frequency variation slope at each range interval during the first reading from the tube to develop the frequency variation of a waveform 730. During the next reading from the tube, a signal 180 degrees out of phase from the waveform 700 develops the frequency variation of a waveform 732 to sweep the data from each range interval with an inverted frequency variation slope, both with the same setting of the step attenuator 270 from the decoder 254. It is to be noted that by providing a correction with a swept oscillator in response to a slope setting during each azimuth scan sector, for example, the frequency correction approaches a continuous sample type of operation. Thus during each reading period, the signal is translated in both directions of increasing and decreasing frequency, one frequency translation being in the correct direction for narrowing the target signal. The video signal is applied from the amplifier 280 to the doppler filter bank 286 where the doppler filter receives the target signal while it is being translated to a narrow spectral width and to a wide spectral width. During the incorrect signal translation, the signal power to noise ratio is relatively low so that essentially no signal is passed to the detectors such as 288 and 290. However, during the time of the correct frequency translation, the spectrally narrow target signal has a large signal to noise ratio and one of the detectors applies a signal to the OR gate 290. Because of the correction of the spectral width, the doppler filters such as 282 and 284 may have desirable narrow bands. The fixed threshold detector circuit 292 passes signals that exceed the threshold to the AND gate 294 which during the blanking pulse period, applies a detected moving target signal to the display device 296. The detected signal may be applied to the intensity grid of a cathode ray tube included in the display device 296. In a range-azimuth display, the target indications such as 740 and 742 have a width determined by the continuance in each range gate of a target signal during reading at each range interval. The analog value of the antenna azimuth angle η_(s) is applied to the A to D converter 303 and to the register 304 during each writing period W₁ or W₂ as defined by the signals S₁ or S₂. The data is then applied through the D to A converter 307 to a horizontal deflection generator (not shown) of the display device 296 to control the horizontal or azimuth position. It is to be noted that the timing signals and the detected video signals may be utilized in other types of devices such as a correlation computer.

As an illustration of the operation of the system in accordance with the invention, the frequency of f_(x) may be 9200 MC, the local oscillator 70 may operate at 9230 MC, f_(o) may be equal to 30 MC, f₁ may be equal to 13.5 MC, f_(ds) may have a range of 10 to 30 KC and (f_(o) - f₁) may be equal to 16.5 MC. The signals on the lead 122 may have a range of DC to 2 MC (for 1/2 microsecond pulses) when the system is operating at a PRF of 2 KC so that the 1/2 PRF point has a frequency of 1 MC. The signal read from the processor on the line 222 may have a frequency range of 0 to 14 MC (corresponding to 1/2 PRF) and the triangular waveform may have a frequency of 50 KC. As the data is recorded on the tube in 140 microseconds and each range interval is read in 10 microseconds, the storage arrangement provides a frequency multiplication factor of 14,000. Thus the signal of the waveform 703 (FIG. 22) is effectively multiplied by a constant and the 1/2 PRF frequency is 14 MC. For the doppler filter bank, 28 filters each of 500 KC bandwidth and at 500 KC centers may be utilized. The target spread of the waveform 703 resulting from the velocity compensation may be in the range of 2.8 MC, for example. A similar frequency region would be encompassed by the clutter spectrum if it were not for the velocity compensation in accordance with the invention, which would result in additional frequency eclipsing of the target signal.

Referring now to FIG. 29, another arrangement is shown of the PRF modulator in accordance with the principles of the invention for developing a 90° or a 45° offset with folding respectively at a one-quarter or a one-eighth PRF point of the spectrum. The input signal is applied from a source 909 on a lead 910 to a cos θ modulator or mixer 912 at a frequency of f₂. A source of frequency stable reference signals at a frequency f₁ may be developed by an oscillator 914. It is to be noted that a phase locked loop may be coupled between the signals on the lead 910 and the oscillator 914 in accordance with the invention. Also, the frequency f₁ may equal f₂ or may be less or greater in accordance with the invention. When f₁ < f₂ or f₁ > f₂, the folding occurs at a point from f₂ equal to the offset frequency such 1/4 or 1/8 PRF. Although the principles of the invention include utilizing the sum frequency at the output 913 of the mixer 912, the difference frequency is modulated to DC as previously explained.

Balanced modulators 918 and 920 which are similar, may be responsive to the PRF synchronizing pulses on a lead 922. The modulator 918 includes a primary winding 919 and secondary windings 921 and 923 respectively having first ends coupled through the anodes and cathodes of diodes 925 and 927 to a lead 926. The lead 926 is coupled to ground through parallel inductor and capacitor to provide summing and a DC voltage source to the transformer secondary winding. The modulator 920 is similar to the modulator 918 and includes diodes coupled to the lead 926. The stable reference signal is applied from the oscillator 914 through a lead 924 to the primary winding of the modulator 918 and through a 90° phase shifter 928 to the primary winding of the balanced modulator 920. A counter and logic circuit 932 responds to the PRF synchronizing pulse to apply signals A and C to the second ends of the secondary windings 921 and 923 of the modulator 918 and signals B and D to the secondary windings of the balanced modulator 920. The counter and logic circuit 932 may include inductors and capacitors coupled to ground from each lead of signals A, B, C and D for providing a low impedance to ground for IF frequencies. The signals developed in the modulators 918 and 920 are applied to the lead 926, a limiter 927 and through a lead 929 to the mixer 912. For a 90° and a 45° phase shift each PRF interval, the modulating arrangement respectively provides frequency offsets of 1/4 and 1/8 PRF on the lead 926 and respective folding to DC of the spectrum at 1/4 and 1/8 PRF points on the lead 913. The counter and logic circuit 932 develops the following signals A, B, C and D at each PRF interval. The binary conductors of A, B, C and D may be +1 and -1 respectively representing conduction and nonconduction of the corresponding diode.

    ______________________________________                                         90° θ SHIFT EACH PRF INTERVAL                                     Phase           Signal States                                                                A    B      C      D                                             ______________________________________                                         0°       1      0      0    0                                           90°      0      1      0    0                                           180°     0      0      1    0                                           270°     0      0      0    1                                           360°     1      0      0    0                                           45° θ SHIFT EACH PRF INTERVAL                                     Relative        Signal States                                                  Phase           A      B      C    D                                           ______________________________________                                         0°       1      0      0    0                                           45°      1      1      0    0                                           90°      0      1      0    0                                           135°     0      1      1    0                                           180°     0      0      1    0                                           225°     0      0      1    1                                           270°     0      0      0    1                                           315°     1      0      0    1                                           360°     1      0      0    0                                           ______________________________________                                    

Counters and diode logic circuits for devloping the above states in response to each PRF synchronizing pulse are of a type well known in the art and will not be explained in further detail. For example, when signals A and C (or B and D) are respectively 1 and 0, + sin ωt and - sin ωt (or + cos ωt and - cos ωt) signals are developed by the windings 921 and 923 to provide + sin ωt output on the lead 926. A 00 condition for signals A and C or B and D opens the diode switches such that no output is applied to the lead 926. Also a 11 condition for signals A and C or for B and D develops a + sin ωt and a - sin ωt (or + cos ωt and - sin ωt) signal at the two transformers to apply a zero output to the lead 926.

Thus the 90 degrees and 45 degrees phase shifts provide spectral folding at 1/4 and 1/8 PRF points with translation of the selected clutter signal (f₁) to 1/4 PRF and 1/8 PRF in the mixer 912. The principles of the invention are applicable to developing spectral folding at any suitable PRF points and are not limited to the illustrated arrangements.

The illustrated system thus operates in a coherent fashion either with a conventional coherent transmitter and receiver or by converting an existing non-coherent system to a coherent system. Although the system operates with highly reliable operation in low or medium PRF radar systems, the principles of the invention are applicable to all types of pulse rates. The principles of the invention are applicable to systems operating with a stable platform, with an inertial guidance system or with a doppler guidance system as well as other velocity determining systems. Although the illustrated system is especially applicable for detecting targets during a search operation, the principles of the invention are applicable to other types of operations. The system of the invention may be operated with multiple PRF selection to overcome the problems of frequency eclipsing of the target signals by the spectral clutter signal as is well known in the art. A velocity compensation feature effectively maintains the clutter signals stationary or eliminates the mean doppler effect due to platform motion. This velocity compensator operates reliably during the absence of clutter signals by a first order linear prediction correction. The IF signal is modulated to a clutter referenced video signal with the clutter shifted to a frequency of 1/2 PRF so that the target frequencies are within the frequency limitations of the storage device. An improved storage arrangement such as two tubes is utilized for alternately storing the video signal in range and azimuth. The video signal is then read over each range interval for a plurality of radar sweeps so that the width of the target may be utilized for reliable detection. After reading the stored signal at a selected rate, the signal is passed through a gated clutter filter so that the clutter is removed with a rejection band corresponding to the width of the filter, as a function of the aircraft velocity and the angle of the antenna line-of-sight. After removal of the clutter, the target signal is frequency translated to the original narrow spectral width prior to the velocity compensation. The matched target signal is then passed through doppler filters and detectors for detecting a moving target and to a display device, for example.

Thus, there has been described an improved synchronous detector system that develops a video signal with a spectral signal modulated to a desired pulse repetition frequency. The system may operate reliably at varying pulse repetition frequency and input signal frequency. In the illustrated system the pulse repetition frequency modulator provides the target signal in a desired frequency range for recording. 

What is claimed is:
 1. A system for frequency modulating an input signal having spectral signals repetitive at a predetermined pulse repetition frequency with a selected spectral signal having a predetermined mean frequency comprising:mixing means responsive to the input signal, a source of frequency stable reference signals at substantially the same frequency of said input signal, and means coupled between said source of reference signals and said mixing means for controlling the phase of the reference signals as a function of the pulse repetition frequency to provide a predetermined frequency offset to the reference signals.
 2. A system for frequency modulating an input signal having spectral signals repetitive at a predetermined pulse repetition frequency with a selected spectral signal having a predetermined mean frequency comprising:mixing means responsive to the input signal, a source of oscillator signals at the predetermined frequency in phase synchronism therewith, phase shifting means coupled between said source of oscillator signals and said mixing means, and means coupled to said phase shifting means for varying the phase at predetermined intervals of said pulse repetition frequency.
 3. A system responsive to an IF signal having a spectrum includng a selected signal at one-half pulse repetition intervals of a predetermined pulse repetition rate and responsive to synchronizing signals at the pulse repetition frequency to develop a video signal with the selected signal at a predetermined noninteger multiple of the pulse repetition frequency comprising:mixing means responsive to the IF signal, oscillator means responsive to said IF signal for being phase locked therewith, phase shifting means coupled between said oscillating means and said mixing means for establishing first or second phases of the signal developed by said oscillator means, and means coupled to said phase shifting means and responsive to the synchronizing signals to change the phase of the oscillator signals during intervals selected as a function of the pulse repetition intervals so that said selected signals appear at said selected noninteger multiples of said pulse repetition frequency.
 4. A system for modulating an IF signal to a video signal, the IF signal being developed from a pulsed radar system transmitting pulses in response to a synchronizing signal at a selected pulse repetition frequency, the IF signal having clutter spectral signals at multiples of the pulse repetition frequency and the video signal having the clutter signal at multiples of one-half pulse repetition frequency comprising:mixing means responsive to the IF signal, a voltage controlled oscillator, a phase detector responsive to said IF signal and the voltage controlled oscillator, a phase locked loop coupled between said phase detector and said voltage controlled oscillator, phase shifting means coupled between said voltage controlled oscillator and said mixing means for developing first and second phase shifts substantially 180 degrees apart, and bistable means coupled to said phase shifting means and responsive to the synchronizing signal to alternately develop the first or the second phase shifts during each succeeding pulse repetition rate interval.
 5. A system for frequency modulating an input signal having spectral signal repetitive at a predetermined pulse repetition frequency with a selected spectral signal having a predetermined mean frequency comprising:mixing means responsive to the input signal, a source of reference signals at substantially the same frequency of said input signal, first and second modulating means coupled to the reference signals, summing means coupled between said first and second modulating means and said mixer for applying reference signals with a predetermined frequency offset thereto, and a source of control signals responsive to the pulse repetition frequency and coupled to said first and second modulating means for controlling the frequency offset applied to said mixing means as a function of the pulse repetition frequency so that the input signal after frequency modulation has the selected spectral signal at a frequency corresponding to said frequency offset. 